Well, I certainly cannot diagnose dementia from a video but if you are correct we will have President Pence soon and he will carry on just fine in my opinion.
I don't know if Trump has dementia or not. But, that's exactly my point -- that didn't prevent people from saying the same about Hillary. But, if Trump had signs of dementia do you trust that any of the sycophants in his cabinet or on his staff would ever admit it? Here is where we have a strong disagreement: I don't believe Trump has accomplished ANYTHING of significance other than damaging institutions and making our politics even more corrosive.
Remember the Ford Plant and the Carrier jobs?
https://www.axios.com/carrier-moving-jobs-trump-vowed-to-save-to-mexico-2446291250.html
There are two ways to look at the recent announcements that these companies are doing what Trump said they weren't going to do. One, he was snookered and they played him (which doesn't say much for Trump's deal making skills), or two, all the announcements that he "cut a deal" and "saved jobs" were simply window-dressing and theater to make it APPEAR as if Trump had taken quick and decisive action and was keeping his promises to save jobs. I'm a firm believer that Trump did have an opportunity to temporarily delay automation, job losses to other countries, and the erosion of employment opportunities for the working class. But, it required that he not just make threats but act on them. So far, Trump's tactic has been to make a controversial threat/tweet, and then "negotiate" a deal that allowed both him and the entity he went after to save face and make it a win-win. However, once a bit of time passed and his adherents were suitably impressed that he was accomplishing amazing things, the entities went on to do what they were going to do anyway. Remember Trump saying China was going to reign in North Korea? They have conducted numerous missile launches since then, and China has actually increased their economic trade with North Korea significantly. Trump's response? He says China tried and that's all that matters. Remember how Trump said China was a currency maniupulator and stealing jobs? After meeting with China's President for 10 minutes he changed his mind. Remember how he was going to build a wall and make Mexico pat for it? Not a single brick has been laid. Now, he's floating a ridiculous plan to put solar panels on the wall in an attempt to fund it and get Democrats on board.
This isn't a naive politician trying to find his way. This is classic Trump in action. Make an outlandish announcement, get the press and media attention, and then quietly settle for almost nothing of what you originally announced. What's the purpose? To convince people of his brilliance and ability. It is the work of a con-artist or a showman who never cares about the results. Trump said it himself in his books. People want to believe. Sell them a dream and sprinkle in what they'd like to see happen. They will buy it.
So what has Trump accomplished?
Travel ban? Blocked.
Qatar/Saudi fight
China pressuring NK? China tried.
China manipulating their currency? I changed my mind.
Comey tapes? Don't have them. I was bluffing.
Carrier/Ford? Doing what they planned to do anyway.
Secret plan to defeat ISIS? More bombing and boots on the ground and a new deployment of soldiers to Afghanistan. Trump said he knew more than the Generals, but he's given them carte blanche to do what they want. There was no secret plan.
Take the oil? No oil has been taken.
Build the wall? Even Ann Coulter believes it will never happen under Trump.
DACA must end? Make DACA permanent and snow people by saying you ended DAPA which was never even fully in effect and largely blocked by the courts.
Hire the best and brightest? Flynn under indictment, Spicer "promoted" to no longer giving briefings, Deputy CoS gone, Kushner being investigated, Reince about to be sent to Greece if he doesn't fix Trump's mistakes by July 4th, Gorka can't get a clearance, etc.
The list goes on and on. What HAS he accomplished other than waging war on the media and signing a few impotent or blocked Executive Orders? He's barely filled a 1/10 of the Executive branch positions his administration needs to effectively operate government.
All I can think of is Gorsuch, and only because McConnell handled that for him.